Piezoelectric actuators are used in a wide variety of technical fields to activate a final controlling element. In particular piezoelectric actuators are suitable for activating an on-off valve of a pump-nozzle unit of a fuel injection system. Piezoelectric actuators can be switched very quickly, so the injection processes of the pump-nozzle unit can be precisely controlled.
Modern pump-nozzle units with which, for example, diesel is injected into an internal-combustion engine of a motor vehicle, use high fuel pressures of up to 2,000 bar. In addition exhaust gas quality requirements are constantly increasing, so very precise adjustment of the injected quantity of fuel and equalization of the quantity injected into different cylinders of an internal-combustion engine are required. The precise injection processes should also be adhered to throughout the life of the pump-nozzle unit even in the case of corresponding ageing phenomena. Tolerances that occur in the case of pump-nozzle units should also be compensated.
These objectives require precise regulation of the pump-nozzle units. In particular the hydraulic cut-off of the pump-nozzle unit, which may be derived from the opening behavior of the piezoelectric actuator, should be determined as precisely as possible for this purpose. Knowledge of the instant of hydraulic cut-off of the pump-nozzle unit is necessary to ensure minimum quantity stability owing to relatively high injection sensitivity during the charging process of the piezoelectric actuator, as is knowledge of its hysteresis behavior. Knowledge of the instant of hydraulic cut-off of the pump-nozzle unit is also necessary for cylinder-individual correction.
Since in the case of injection valves there is in general no position measuring at the injection valve however, the opening behavior of the injection valve is determined from the voltage of the piezoelectric actuator. Various control methods are known for this purpose.